To evaluate the performance of the distributed medium access control layer of the emerging ultrawideband and 60-GHz millimeter wave (mmWave) wireless personal area networks based on ECMA-368 and ECMA-387 standards, the first step is to determine the beacon period length (BPL) of the superframe in a given network. In this paper, we provide an analytical model for the probability mass function (PMF) of the BPL as a function of the network dimensions, number of beaconing devices, antenna beamwidth, and the transmission range of the devices. To enable devices with steerable directional antennas in the ECMA-387 standard to have simultaneous communications with neighbors in their different antenna sectors, we propose an improvement to the standard for which we computed the PMF of the BPL in its worst case. The effect of beacon period (BP) contraction on the PMF is also considered and modeled. The proposed model for all cases is evaluated by simulating different scenarios in the network and the results show that on average, the model for the average BPL has an error of 1.2 and 2.5 percent in the current definition of the standard and in the proposed modification, respectively, without BP contraction and 0.9 and 1.5 percent, respectively, with BP contraction.